In recent years, with improvement of technology in image input devices such as digital cameras and scanners, resolutions of image data input via these input devices are continuously increasing. With image data of low resolution, which has small amount of image data, there has been no trouble in working such as transmitting, storing, displaying and editing. However, as the resolution increases, the amount of image data becomes enormous and causes the problems that it takes a long time to transmit the data or that a large memory capacity is required for storing the data. As a method for transmitting and displaying image data efficiently, progressive transmission of image data is drawing attention. This method transmits low quality image data in an early phase of image data transmission so that the image may be roughly grasped, and the image quality is increased with proceeding of data transmission. JPEG (Joint Photographic Experts Group), an international standard of encoding method for still images according to ITU-T Recommendation T. 81 and ISO/IEC 10918-1 prescribes progressive transmission in its expanding method.
When transmitting image data phase-by-phase according to the conventional progressive transmission method, if the image data to be transmitted is a natural image, the receiving side can roughly grasp the image based on a decoded image of low image quality obtained in an early phase of transmission. However, if the to-be-transmitted image is a document image including characters or lines, it is often difficult to obtain information from the characters or lines included in the image though an general configuration of the image can be comprehended based on the decoded image of low image quality.
For example, if image data in which multi-level image data and bi-level image data are mixed, such as map information, is transmitted phase-by-phase, it is difficult to read bi-level information from the decoded image of low quality in many cases.
The present invention is made in view of the above-described problems and its object is to make it possible, in the case where encoded data is generated by encoding image data including mixed multi-level image data and bi-level image data and the encoded data is then decoded, to read bi-level image data from the decoded image in spite of its low resolution.